1. Field of the Invention
This invention relates to a system for and method of aligning two bodies, and more particularly to a system for and method of aligning a mask and a waver in a mask aligner used in the manufacture of semiconductor integrated circuits.
2. Description of the Prior Art
To align a mask and a wafer, there is known a system in which two or three groups of alignment marks are provided on the mask and wafer, respectively, and these alignment marks are scanned by an emitted beam such as a laser beam or the like and the light energy scattered by the alignment marks is received and converted into an electrical signal stream. There is also known a system in which the images of alignment marks are picked up by a photosensor array or an image pickup tube of a television and the signals thereof are processed to obtain an alignment signal.
U.S. Pat. No. 4,167,677 describes that the alignment marks of a mask and a wafer are scanned by a laser beam or the like and the scattered light resulting therefrom is photoelectrically converted into electrical pulses and the positions of these pulses are measured by a counter or the like to thereby effect alignment.
For example, alignment marks M such as shown at (a) in FIG. 1 of the accompanying drawings are depicted on a mask while marks W such as shown at (b) in FIG. 1 are depicted on a wafer, and these alignment marks M and W of the mask 1 and wafer 2 are scanned by a laser beam L in a device of the construction as illustrated in FIG. 2 of the accompanying drawings. The mask 1 and wafer 2 are finally aligned relative to each other, such as shown at (c) in FIG. 1. In this case, the wafer 2 placed on the stage 3 of FIG. 2 may usually be displaced relative to the mask 1 as shown, for example, (d) in FIG. 1 before the aligning is effected. In this condition, when the alignment marks M and W of the mask 1 and wafer 2 are scanned in the direction of arrow by the laser beam L emitted from a laser light source 6 through a deflector 4 comprising a polygon mirror or the like and a beam splitter 5, both shown in FIG. 2, the scattered light thereof travels back along the original optical path, passes through the beam splitter 5 and arrives at a photoelectric detector 8 through a condenser lens 7, whereby pulse signals are shown at (e) in FIG. 1 are obtained by the detector 8. A control circuit 9 of FIG. 2 cuts these pulse signals by a suitable threshold voltage V by means of a comparator 10, such that from the pulse train shown at (f) in FIG. 1, the spacing between the alignment marks M and W is found and the amount of relative displacement of the mask 1 and wafer 2 is determined.and l.sub.2 from the opposite edges of the line 13 of the mark M, W as enlargedly shown in FIG. 3 of the accompanying drawings, and the sum l.sub.3 has a time expanse larger than the width a of the actual mark. Therefore, when the alignment marks M and W have come close to each other as shown in FIG. 4(a) of the accompanying drawings, the output of the detector 8 includes a portion in which the signals are superposed one upon the other like the waveform w shown in FIG. 4(b) of the accompanying drawings. In such condition, a proper positional relation cannot be found, and thus aligning is difficult. Heretofore, in such a case, the step of moving the mask 1 or the wafer 2 by trial and error until the pulse signal is separated into six or the step of increasing the threshold voltage V to V' up to a level whereat the pulse signal is separated has been necessary.
However, if an attempt is made to make the alignment marks M and W small, the probability with which the alignment marks M and W of the mask 1 and wafer 2 come close to each other or overlap each other as shown in FIG. 4(a) during the setting of the wafer 2 becomes very high. Accordingly, if the marks M and W are made excessively small, there arises a problem that the by-trial-and-error driving is repeated and much time is required for aligning, or aligning cannot be accomplished. This forms a great barrier when the alignment marks M and W are to be made small.